This invention relates to asphalt compositions. More specifically, this invention relates to rubber-modified asphalt compositions with an improved softening point.
Asphalt has become an important material for building and repairing roads. It is also used in applications such as roofing and weather sealing. The estimated annual use of asphalt in the United States alone is over 13 million kilograms.
The addition of rubber to asphalt can improve properties such as crack resistance, softening points, viscosity, tensile strength, elongation, toughness, and tenacity. It has also been shown that curing agents can be used to further improve asphalt properties. Curing agents often used in asphalt compositions are phenolic resins and elemental sulfur. However, phenolic resins require long curing times and elemental sulfur must typically be used in large quantities.
It would therefore be desirable to find a better curing agent for rubber-modified asphalt compositions which is capable of overcoming the above-mentioned problems of typical curing agents while still improving the physical characteristics (e.g. softening point) of asphalt compositions.
In one embodiment, an asphalt composition is provided. The asphalt composition includes at least a rubber, asphalt, and a bismaleimide curing agent. The rubber includes at least polydiene, and may include additional monomer units such as vinyl-substituted aromatic hydrocarbons. In another embodiment, a method for producing an asphalt composition is provided.
Asphalt compositions containing rubbers cured with bismaleimides exhibit beneficial properties. For example, the use of a bismaleimide curing agent results in improvements to the softening point of the asphalt composition while maintaining desirable tenacity levels. As known to the skilled artisan, tenacity levels represent the asphalt""s strength, toughness, and durability, and is a measure of the resilience of the asphalt at room temperature. The softening point of the present asphalt compositions is between about 35 and 120xc2x0 C., preferably between about 45 and 95xc2x0 C.
The present asphalt composition includes at least a rubber, asphalt, and a bismaleimide curing agent. The rubber is preferably a polydiene, and may include additional monomer units such as vinyl-substituted aromatic hydrocarbons.
The preferred rubber is at least a polydiene. Exemplary diene contributed monomer units include 1,3-butadiene, isoprene, 2,3-dimethyl-1,3-butadiene, 1,4-pentadiene, and 1,3-pentadiene. Preferred diene contributed monomer units are 1,3-butadiene and isoprene. The rubber may include monomer units contributed from more than one diene molecule. For example, the rubber may be a poly(1,3-butadiene-co-isoprene).
In addition, the rubber may also contain additional monomer contributed units. Exemplary monomer contributed units include vinyl-substituted aromatic hydrocarbons. Suitable vinyl-substituted aromatic hydrocarbons include styrene, xcex1-methylstyrene, 1-vinylnphthalene, 2-vinylnaphthalene, 1-xcex1-methyl vinylnaphthalene, 2xcex1-methyl vinylnaphthalene, as well as alkyl cycloalkyl, aryl, alkaryl, and aralkyl derivatives thereof, and di-or tri-vinyl-substituted aromatic hydrocarbons. A preferred vinyl-substituted aromatic hydrocarbon is styrene.
The rubber is preferably any of styrene-butadiene diblock polymers, as well as any styrene-butadiene, polybutadiene, polyisoprene, styrene-isoprene block or random polymers, styrene-butadiene block or random polymers, and mixtures thereof The rubber compositions may optionally be vinyl-modified to create high vinyl conjugated diene monomer units. A 1,2-microstructure controlling agent or randomizing modifier may be used to control the vinyl content in the conjugated diene contributed monomer units. Suitable modifiers include hexamethylphosphoric acid triamide, N,N,Nxe2x80x2,Nxe2x80x2-tetramethylethylene diamine, ethylene glyclol dimethyl ethyer, diethylene glycol dimethyl ether, triethylene gylcol dimethyl ether, tetraethylene glycol dimethyl ether, tetrahydrofuran (THF), 1,4-diazabicyclo [2.2.2] octane, diethyl ethyer, triethylamine, tri-n-butylphosphine, p-dioxane, 1,2-dimethyoxy ethane, dimethyl ether, methyl ethyl ether, ethyl propyl ether, di-n-propyl ether, di-n-octyl ether, anisole, dibenzyl ether, diphenyl ether, dimethylethylamine, bis-oxalanyl propane, tri-n-propyl amine, trimethyl amine, triethyl amine, N,N-dimethyl aniline, N-ethylpiperidine, N-methyl-N-ethyl aniline, N-methylmorpholine, tetramethylenediamine, oligomeric oxolanyl propanes, 2,2-bis-(4-methyl dioxane), and bistetrahydrofuryl propane. One or more vinyl modifiers may be used. As the modifier charge increases, the percentage of 1,2-microstructure (vinyl content) increases in the conjugated diene contributed monomer units. Vinyl-modified conjugated diene polymers and trans-cis conjugated diene polymers are equally applicable in the present invention.
The rubber compositions may also be further functionalized to contain different organic functional groups useful in the formation of the present asphalt composition. Suitable functional groups include carboxy, hydroxy, formyl, amino, halo, and alkyl. Amino functionalized rubbers are especially preferred due to their increased ability to interact with bismaleimides.
The rubber composition is preferably cured with a bismaleimide. Importantly, however, the present invention contemplates curing the rubber with a bismaleimide prior to or after combining it with asphalt. Suitable bismaleimides for use as rubber curing agents are represented by either of the following formulas: 
wherein R1 may be an aromatic group, a aliphatic group, a cycloaliphatic group, or an aliphatic group containing siloxane. However, any bismaleimide known to the skilled artisan may be employed inthe present asphalt compositions.
Bismaleimides may be prepared by a variety of techniques. One method of preparation entails formation of the maleamic acid via reaction of the corresponding primary amine with maleic anhydride, followed by dehydrative closure of the maleamic acid with acetic anhydride.
A variety of other approaches may also be employed. For example, dicyclohexylcarbodiimide (DCC) closes maleamic acids. With DCC, the product is exclusively isomaleimide. However in the presence of suitable isomerizing agents, such as 1-hydroxybenzotriazole (HOBt), the product is almost solely the maleimide. The function of the HOBt could allow the closure to proceed via the HOBt ester of the maleamic acid (formed via the agency of DCC) which presumably closes preferentially to the maleimide.
Suitable bismaleimides formed by the outlined methods or available commercially include N,Nxe2x80x2-ethylene-bis-maleimide, N,Nxe2x80x2-butylene-bis-maleimide, N,Nxe2x80x2-phenylene-bis-maleimide, N,Nxe2x80x2-hexamethylene-bis-maleimide, N,Nxe2x80x2-4,4xe2x80x2-diphenyl methane-bis-maleimide, N,Nxe2x80x2-4,4xe2x80x2-diphenyl ether-bis-maleimide, N,Nxe2x80x2-4,4xe2x80x2-diphenyl sulfone-bis-maleimide, N,Nxe2x80x2-4,4xe2x80x2-dicyclohexyl methane-bis-maleimide, N,Nxe2x80x2-xylylene-bis-maleimide, N,Nxe2x80x2-diphenyl cyclohexane-bis-maleimide, N,Nxe2x80x2-(m-phenylene)bismaleimide, N,Nxe2x80x2-(p-phenylene)-bismaleimide, N,Nxe2x80x2-(p-tolylene)bismaleimide, N,Nxe2x80x2-(methylenedi-p-phenylene)-bismaleimide, N,Nxe2x80x2-(oxydi-p-phenylene)bismaleimide, xcex1,xcex1,-bis-(4-phenylene)-bismaleimide, N,Nxe2x80x2-(m-xylylene)bis-citraconimide, xcex1,xcex1-bis-(4-maleimidophenyl)-meta-diisopropylbenzene, and mixtures thereof.
Asphalts, as the term is use herein, include cementitious materials in which the predominating constituents are bitumens that occur in nature or are obtained in petroleum processing. Bitumen is a term which encompasses cementitious substances, natural or manufactured, composed principally of high molecular weight hydrocarbons, of which asphalts, tars, pitches, and asphalitites are typical. Asphalts are often classified as solids, semisolids, or liquids. They are often defined as the manufactured materials that are produced during petroleum processing. Asphalts characteristically contain very high molecular weight molecular polar species, called asphaltenes, which are soluble in carbon disulfide, pyridine, aromatic hydrocarbons, chlorinated hydrocarbons, and THF. Asphalts produced from the refining of petroleum have been used primarily in paving and roofing applications.
When asphalt polymer blends are prepared for use, given portions of polymer are blended into the molten asphalt at a specified temperature in conjunction with, or prior to, the addition of a curing agent. The rubber is dissolved in molten asphalt at temperatures greater than about 120xc2x0 C. The rubber may be added to the asphalt in a range of about 1 to 50 parts by weight (pbw) rubber per 100 pbw asphalt, preferably in a range of about 2 to 25 pbw, and most preferably in a range of about 4 to 14 pbw. After the rubber has fully dissolved, at least one bismaleimide curing agent is added to the composition. Alternatively, the curing agent(s) may be added to the molten asphalt simultaneously with the rubber. The curing agent is preferably added in amounts between about 0.5 and 3 wt % of the total composition, more preferably between about 0.25 and 2 wt % of the total composition. After mixing, for example between about 25 and 400 minutes, more preferably between about 50 and 300 minutes, a homogeneous mixture is obtained. The mixing temperature is preferably between about 145 and 205xc2x0 C., most preferably between about 160 and 193xc2x0 C.